The present invention relates to an optical head to be used for read/write of optical recording media having different substrate thicknesses, such as DVD (Digital Video Disk) and CD (Compact Disk).
In optical disk devices employed for these optical recording media, there is used an optical head which focuses a laser beam on a micro spot of a recording face through a transparent substrate protecting the recording face wherein preciseness of the micron order is required.
In these optical disk devices where the convergence beam passes through a parallel plate of the transparent substrate, a wave front aberration is caused depending on thickness of the parallel plate. Here, the wave front aberration means variance of iso-phase wave front from true sphere. If the variance becomes so large that the focussing spot spreads wider than the diffraction limit, sufficient read/write performance cannot be obtained. Therefore, object lenses employed for the optical heads are generally designed to compensate for the wave front aberration, for obtaining a focussing spot comparable to the diffraction limit.
However, as above described, there are optical disks designed to have different substrate thicknesses according to their usage, such as DVD and CD. When the substrate thickness differs, compensation value of the wave front aberration should differ, resulting in that an optical head designed for a specific substrate thickness cannot read/write optical disks having different substrate thickness because of unmatched compensation values of the wave front aberration.
For dealing with the above problem, a method of restricting lens aperture is proposed in Extracted Abstracts of the 56th Autumn Meeting, 1995 of the Japan Society of Applied Physics, 29a-ZA-6, p.956.
FIG. 3 is a perspective view illustrating the prior proposal, wherein a laser light from a semiconductor laser 11 through a half-mirror 12 and a shading circle 13 is converged by an object lens 14 on a micro spot of a recording face of an optical disk 15. Reflected laser light from the optical disk 15 is separated by the half-mirror 12 to be detected by a photo-detector 16.
In front of the object lens 14, the shading circle 13 having a circular aperture is provided for restricting the light beam from the semiconductor laser 11. The shading circle 13 shades the light beam passing through the periphery of the object lens 14 wherein the aberration is comparatively large.
By thus eliminating the effect of the wave front aberration, reproduction of optical disks having different substrate thicknesses is enabled in the prior proposal.
However, the center of the shading circle 13 must be arranged on the same axis with the center of the object lens 14, in the method of the prior proposal, which needs two-dimensional positioning of the shading circle 13 on a plane parallel to the object lens 14. The two-dimensional positioning makes assembling of the optical head somewhat difficult, and may be accompanied a positioning error causing invalid shading effect.
Furthermore, there is another problem in that the shading circle 13 must be shifted unison with the object lens 14 when the object lens 14 is shifted in a radial direction of the optical disk 15, perpendicular to the recording track, for fine tracking of the recording face in read/write operation of the optical disk 15, which requires a somewhat larger shifting mechanism, hindering high-speed operation as well as miniaturization of the optical head.